Background: The U.S. Army Corps of Engineers (Corps) and Idaho Water Resource Board (IWRB) are partnering on an interim feasibility study of the lower Boise River and tributaries from Lucky Peak Dam downstream to the confluence with the Snake River. Significant development in the river corridor and population growth has sparked interest in flood risk management and water supply issues. Interest has also been expressed in environmental restoration, to include habitat preservation, aesthetics and recreation along the Boise River.

The interim feasibility phase will 1) evaluate and document existing conditions on the Boise River, 2) evaluate and update information about flood risk, 3) analyze surface water storage opportunities in the basin, and 4) develop a path forward to complete the feasibility study. A later, second phase of the feasibility study will conduct a more comprehensive analysis of the watershed to examine additional strategies to address flood risk, water supply and ecosystem restoration purposes.

The Corps developed a two-phased feasibility study approach to assist with the Treasure Valley Comprehensive Aquifer Management Plan (CAMP), a concurrent but separate planning effort initiated by the IWRB to address future water supply and demand issues in the lower Boise River basin over the next 50 years. A later, second phase of the feasibility study will conduct a more comprehensive analysis of the watershed to examine additional strategies to address flood risk, water supply and ecosystem restoration purposes.

You can read the most current fact sheet for an update on study status.

Sponsor: In May 2009, the Corps and the IWRB entered into an agreement to initiate the first phase of a two-phased feasibility study. The IWRB has agreed to cost share the Interim Feasibility Study. The Idaho Department of Water Resources will provide project coordination and other staffing assistance on behalf of the IWRB.

Authority: The Corps' study authorization is provided by Section 414, Water Resources Development Act (WRDA) of 1999, authorizing a feasibility study for flood control on the Boise River; and Section 4038, WRDA 2007, modifying the 1999 authority to include ecosystem restoration and water supply as project purposes.

Study Status: The interim feasibility study is currently focusing on water storage as one potential measure for addressing water supply demand and flood risk reduction planning objectives. The study is currently evaluating surface water storage sites upstream of Lucky Peak Dam and updating flood risk information.

In August 2010, the study team completed a surface water storage screening analysis that scored sites for six criteria, including future water demand, flood risk reduction, hydropower potential, a relative cost index, and social and environmental effects. The three top-ranked water storage and flood risk reduction sites included (1) replacement of Arrowrock Dam, (2) construction of a new dam at the Alexander Flats site, and (3) construction of a new dam at the Twin Springs site. The 2010 Water Storage Screening Analysis document describes the screening criteria, the process used to score the surface water storage sites and the analysis results.

In October 2011, the Corps completed a preliminary analysis of the Arrowrock site. The purpose of the analysis was to

• Identify the most appropriate surface water storage concept for the Arrowrock site ( i.e., raise the existing structure or construct a new facility downstream),
• Determine whether there were major engineering or geological constraints that would make a dam raise or a new downstream dam technically unfeasible or cost prohibitive, and
• Identify any significant issues that should be considered for future analyses.

The analysis relied on available data, including a review of historical documents related to the planning and construction of Arrowrock Dam, and geological records and maps. A field reconnaissance was conducted from Arrowrock Dam downstream. The Corps coordinated with Reclamation engineers and geologists. (Arrowrock Dam was constructed by and is operated by Reclamation.) The Preliminary Evaluation of the Arrowrock Site report provides more information about the preliminary analysis.

For Information: Questions or comments on the Lower Boise River Interim Feasibility Study may be submitted to:

Boise GI Manager,
US Army Corps of Engineers,
Boise Outreach Office,
720 Park Boulevard, Suite 255
Boise, Idaho 83712
PH: 208-345-2065
Fax: 208-345-2263

Email to: Boise.Office@usace.army.mil